Are you asking about ovulation? If so, count 14 days from the first day of your last period - that's if you're on a 28 day cycle. If you're on a longer cycle, count 14 days before your next one is due.

I think you also need to know that sperm can survive in your body for 5 days before the egg is produced, but the egg can only be fertilized for 24 hours after release. If you pay attention to your v____al discharge during the month, you will notice you have slippery mucus during your fertile days. It's like the white of an egg, and you can stretch it an inch or more between your thumb and forefinger.